Electrically conductive thick film paste compositions composed of a base metal such as copper or nickel, an inorganic binder, dispersed in an organic binder are widely used as terminal electrode materials for multilayer ceramic capacitors. The conductive pastes are fired in a neutral or reducing atmosphere to suppress oxidation of the constituent metals and the internal electrodes, and thus achieve a suitable terminal junction state. The inorganic binder used is mainly glass frit. The flowability (softening behavior) of the glass frit is one of the main factors affecting the sintering behavior of the conductive paste. The conventional base metal conductive pastes for terminal electrodes on multilayer ceramic capacitors contain a glass frit containing lead oxide and zinc oxide as the inorganic binder because for good flowability. Most such pastes have a firing temperature of at least 800° C. so as to provide the required properties after firing. Recent attention has been to conductive paste compositions for forming terminal electrodes of mutilayer ceramic capacitors of which layers are increased and are firable at a lower temperature which results in avoidance of adverse effect on the capacitor. There is also a strong market demand for lead, bismuth and cadmium-free terminal electrode forming material with other requisite properties from use in terminal electrode of multilayer ceramic capacitor.
However, problems associated with conventional conductive compositions containing such lead silicate glass being commonly used in this field when after drying to remove organic binder, fired at a temperature of less than 800° C. and for times sufficient to sinter the inorganic materials and to produce conductor patterns adherent to a terminal electrode-forming site of multilayer ceramic capacitor.
The glass flowability in the electrode is insufficient, compromising adhesion to the capacitor assembly, and even if adhesion to the capacitor assembly is good, sintering of the terminal electrode is inadequate and a dense structure is not formed. As a result, in the subsequent plating step, the plating solution passes through the terminal electrode, causing erosion of the glass components by acid in the solution, resulting in a serious loss in terminal strength.
Selecting a glass frit having a good flowability or low softening point for proceeding with sintering of the electrode at less than 800° C. so as to increase a density of formed electrode causes the glass to begin flowing before the bonding associated with alloying of the conductor in the internal electrode and the conductor in the terminal electrodes resulting into forming a thin dielectric layer between the internal electrode and the terminal electrode, reducing the stability and reliability of the electrical characteristics. Moreover, compositions containing a glass inorganic binder having a lower glass softening point, which also raises the coefficient of thermal expansion, causing cracks in the capacitor assembly or the electrodes because of differences in coefficient of thermal expansion between the electrode and the capacitor assembly, leading to a loss of electrical characteristics, adhesion and reliability.
The industry needs a lead, cadmium and bismuth-free terminal electrode composition containing glasses having a suitable softening point and a coefficient of thermal expansion for firing at a relatively low temperature and with good acid resistance.
In view of the above, it is an object of the invention to provide paste compositions for terminal electrodes, which paste compositions contain a glass frit that is free of lead, cadmium and bismuth and can provide necessary and sufficient post-firing characteristics when fired at a temperature of lower than 800° C., and preferably lower than 750° C.; that is, a glass frit having a softening point and a coefficient of thermal expansion within acceptable ranges and good acid resistance.
This invention enables the design and supply of base metal conductive pastes for terminal electrodes in multilayer ceramic capacitors, which pastes are free of lead, cadmium and bismuth and can be fired at temperatures below 800° C.